1. Field of the Invention
The present invention relates to a deposited film forming method for continuously forming deposited film on a long-size substrate by plasma CVD method, and a deposited film forming apparatus therefor, and particularly to a deposited film forming method and a deposited film forming apparatus for forming deposited film having a composition distribution provided in a direction of film thickness.
2. Related Background Art
In recent years, the demand for the electric power has been increasing worldwide, so that the production of the electric power has been more active made, but with the growth of production comes problems associated with thermal power generation or atomic power generation such as environmental pollution. In these situations, the solar cell generation utilizing the sun rays has been noted because it does not cause any problems such as environmental pollutions or global warming, with fairer distribution of the resource of the solar rays, and it can meet the demand for the electric power which will further increase in the future.
To put the solar cell generation to practical use, it is required that the solar cell has a sufficiently high photoelectric conversion efficiency, as well as excellent characteristics and a stability, and is suitable for mass production. Due to the larger electrical scale generation, a larger solar cell will be required. For these reasons, an amorphous silicon type solar cell has been proposed in which a semiconductor thin film made of an amorphous silicon is deposited on a substrate made of a glass or metallic sheet which is relatively cheap by decomposing a readily available source gas such as silane by glow discharge. This amorphous silicon type solar cell has been noted because it can provide an improved mass productivity and a lower cost, compared with a solar cell made of a single crystal silicon, for which various fabrication methods therefor have been proposed.
In the solar cell generation, unit modules of the solar cell are interconnected in series or parallel to form a unit so that a desired current or voltage is obtained. It is required that in each unit module, no disconnection or short-circuit may occur, and there is less dispersion in the output voltage or output current between unit modules. For this purpose, the uniformity in the characteristics of a semiconductor layer is the most determinative factor in the step of fabricating the unit module. Also, to facilitate the module design and simplify the assembling process of module, the ability to form a semiconductor deposited film having excellent characteristics over a large area can provide a greater mass production of the solar cell and yield a great reduction of production cost.
The semiconductor layer which is an important component of the solar cell contains semiconductor junctions such as pn junction or pin junction. Such a semiconductor junction is formed by sequentially laminating semiconductor layers having different conduction types, or ion implanting or thermally diffusing a dopant having a different conduction type into the semiconductor layer of a certain conduction type. In fabricating the amorphous silicon type solar cell as above described, it is well known that a source gas containing an element as a dopant such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) is mixed into a main source gas such as a silane gas. The mixed source gas is decomposed by glow discharge or the like to obtain a semiconductor film having a desired conduction type, and the semiconductor film is sequentially laminated on a desired substrate, so that a semiconductor junction is easily obtained. Thus, in fabricating an amorphous silicon type solar cell, it is common that by providing a separate film forming chamber corresponding to each semiconductor layer, each semiconductor layer is formed within this film forming chamber.
A deposited film forming method relying on the plasma CVD suitable for the fabrication of such an amorphous silicon type solar cell has been disclosed in U.S. Pat. No. 4,400,409 with regard to the roll to roll mode. This deposited film forming method is such that a plurality of glow discharge regions are provided, a long-size strip-like substrate is disposed along a passage penetrating through those glow discharge regions in sequence. Semiconductor layers having necessary conduction types are deposited in respective glow discharge regions separately provided while the strip-like substrate is being continuously conveyed in a longitudinal direction thereof. Thereby, the solar cell having a desired semiconductor junction can be formed consecutively. Note that in this deposited film forming method, to prevent a dopant gas for use in each glow discharge region from diffusing or mixing into other glow discharge regions, the glow discharge regions are separated from each other by a slit-like separation passage called as a gas gate, in which separation passage there is a flow of a scavenging gas such as Ar or H.sub.2. With such a constitution, the deposited film forming method according to the roll to roll mode can be suitably applied to the fabrication of semiconductor elements in the solar cell.
On the other hand, an attempt for improving the photoelectric conversion efficiency of an amorphous silicon type solar cell has been made in which it has been found that when a group-IV alloy semiconductors such as a-SiGe:H, a-SiGe:F, a-SiGe:H:F, a-SiC:H, a-SiC:F, a-SiC:H:F is used as the i-type (intrinsic) semiconductor layer, the forbidden band width (band gap: E.sub.g.sup.opt) of this i-type semiconductor layer is appropriately changed continuously in a direction of film thickness from the incident side of light, so that the open voltage (V.sub.oc) or curve factor (fill factor: FF) can be greatly improved (20th IEEE PVSC. 1988, "A Novel Design for Amorphous Silicon Solar Cells", S. Guha J. Yang, et al.).
However, the deposited film forming method as above described has a drawback. The large area deposited film can not be uniformly formed even though the composition is continuously changed in the direction of film thickness to change continuously the band gap. This drawback is specifically described in the following.
In the deposited film forming method according to the roll to roll mode as above described, the deposited film is formed while the strip-like substrate is being continuously moved, whereby the formation of the deposited film on the substrate is carried out while the substrate passes through the glow discharge regions. Accordingly, the film thickness of the deposited film can be controlled relatively easily by the deposition rate and the passing speed through the glow discharge regions. On the other hand, in order to provide a composition distribution in a direction of film thickness, it is necessary to provide a distribution in a moving direction of the substrate in a film forming atmosphere within the glow discharge region, because the substrate is moved continuously. However, it is difficult to provide repetitively such a distribution in the film forming atmosphere which depends on the composition or pressure of source gas, or the energy density of glow discharge. Also, in a conventional deposited film forming method with the substrate fixed therein, to provide the distribution in the film forming atmosphere was not conducted, because to continuously change the forbidden band width in a direction of film thickness would impair the uniformity of the deposited film.